The xylene isomers are produced in large volumes from petroleum as feedstocks for a variety of important industrial chemicals. The most important of the xylene isomers is para-xylene, the principal feedstock for polyester, which continues to enjoy a high growth rate from large base demand. Ortho-xylene is used to produce phthalic anhydride, which supplies high-volume but relatively mature markets. Meta-xylene is used in lesser but growing volumes for such products as plasticizers, azo dyes and wood preservers.
A prior art aromatics complex flow scheme has been disclosed by Meyers in Part 2 of the Handbook of Petroleum Refining Processes, 2d. Edition, in 1997 published by McGraw-Hill.
In general, a xylene production facility can have various types of processing reactions. One is a transalkylation in which benzene and/or toluene are reacted with C9+ aromatics to form more methylated aromatics. Another is xylene isomerization, which may also include dealkylation, where a non-equilibrium mixture of xylenes is isomerized. And another is the disproportionation of toluene to yield one mole of benzene per mole of xylene produced.
In the transalkylation process, adverse side reactions can occur. For instance, the aromatic ring may become saturated or even cleaved resulting in naphthene co-production. The co-production of these non-aromatics, of course, results in a loss of valuable aromatics. Moreover, benzene is often a sought co-product from a xylene production facility. As some of the naphthenes have similar boiling points to benzene, they are not readily removed to achieve a benzene product of sought purity for commercial applications which frequently demand a benzene product having at least a 99.85 percent purity.
Accordingly, a need exists for catalysts and processes for the transalkylation of alkylaromatics, which processes have desirable selectivity of conversion to the desired alkylaromatics such as xylenes, yet at sufficiently high rates of conversion to be commercially feasible.
U.S. Pat. No. 3,562,345 (Mitsche) discloses catalysts for transalkylation or disproportionation of alkylaromatics comprising aluminosilicates such as mordenite. Catalytically active metals such as groups VIB and VIII metals may be present.
U.S. Pat. No. 3,951,868 (Wilhelm) discloses catalysts for hydrocarbon conversions comprising a platinum group metal and indium with optionally a Group IVA component such as germanium or tin.
U.S. Pat. No. 4,331,822 (Onodera, et al.) discloses catalysts for xylene isomerization containing molecular sieve such as ZSM-5, platinum and at least one metal from the group of titanium, chromium, zinc, gallium, germanium, strontium, yttrium, zirconium, molybdenum, palladium, tin, barium, cesium, cerium, tungsten, osmium, lead, cadmium, mercury, indium, lanthanum, beryllium, lithium and rubidium.
U.S. Pat. No. 5,847,256 (Ichioka et al.) discloses a process for producing xylene from a feedstock containing C9 alkylaromatics with ethyl-groups over a catalyst containing a zeolite component that is preferably mordenite and with a metal component that is preferably rhenium.
U.S. Pat. No. 6,060,417 (Kato, et al.) discloses catalysts and processes for transalkylation of alkylaromatics wherein the catalysts comprise mordenite, inorganic oxide and/or clay and at least one metal component of rhenium, platinum and nickel. See also, U.S. Pat. No. 6,359,184 (Kato, et al.).
U.S. Pats. Nos. 6,150,292 and 6,465,705 (Merlen, et al.) disclose a xylene isomerization process using a mordenite-containing catalyst that further contains at least one platinum group metal and at least one metal from group III of the periodic table such as gallium, indium or thallium and optionally at least one metal from group IV of the periodic table such as germanium, tin or lead.
U.S. Pats. Nos. 6,613,709 and 6,864,400 (Merlen, et al.) disclose a transalkylation catalyst containing zeolite NES and at lease one metal selected from group VIIB, group VIB and iridium with optionally at least one metal selected from groups III and IV of the periodic table, preferably indium and tin.
U.S. Pat. No. 6,855,854 (James, Jr.) discloses a process using two transalkylation catalysts to react C9+ aromatics with benzene. The catalyst comprises zeolite and optional metal components such as IUPAC 8-10 metal and modifier such as tin, germanium, lead, indium, and mixtures thereof.
U.S. Pat. No. 6,872,866 (Nemeth, et al.) discloses a liquid phase xylene isomerization process which uses a zeolite beta and pentasil-type zeolite. The catalyst can contain a hydrogenation metal component such as a platinum group metal and modifiers such as rhenium, tin, germanium, lead, cobalt, nickel, indium, gallium, zinc, uranium, dysprosium, thallium, and mixtures thereof.